A common element in most electronic equipment is the printed circuit board which comprises a laminated board on which are mounted electronic components such as resistors, capacitors, transistors, integrated circuits, and the like. Alternate layers of the laminated board are etched with copper patterns and electrically insulating layers of such materials as epoxy glass. The etched copper patterns serve as electrical interconnections between mounting holes that are provided for receiving the leads and terminals of electronic components. After the components are mounted, they are soldered in place.
In the manufacture of printed circuit boards, such as the well known "two-sided" board, the etching of the copper patterns is followed by a drilling operation involving the drilling of all of the holes needed for the mounting of the electronic components. On a typical printed circuit board, there may be hundreds of electrical components mounted thereon, each with two or more leads for utilizing integrated circuits involving a variety of hole sizes.
In order to reduce the manual labor and the cost involved in the drilling operation, and in order to assure the required level of accuracy, the drilling operation is usually automatically implemented by a computer-controlled machine operated by a program on a magnetic tape or other digital data storage medium which automatically places the proper hole sizes in all the required locations. The same equipment is also frequently employed for the "routing" of the circuit board, i.e. the cutout of the board at its perimeter from a larger etched board containing a number of identical patterns.
In such automatic drilling equipment, the design of the chuck or bit holder is critical for the proper and cost-effective operation of the equipment. During the process of drilling a single board, the drilling machine might have to change bit sizes several times. The changing of the bit occurs automatically. Responding to programmed controls, one bit is removed and replaced by a bit of a different size. This involves very rapid motion and frequently undesired stresses and impacts are experienced by the bit holder, particularly if the machine motion is not as accurately controlled as it should be. The bit holder must therefore be of a very sturdy and yet resilient construction so that it can withstand such difficult use. In addition, its design must take into account the inevitable failure or destruction of the bit holder after a reasonable period of use and it must accommodate the easy and rapid replacement of the broken holder by a replacement part. Furthermore, for greatest economy, the design of the bit holder should take into account the impacts and stresses to which the holder is frequently subjected, and provision should be made for a minimization of the damage such impacts produce so that repair does not always involve the replacement of expensive parts, but might require only a reassembly of the parts already in use.
Bit holders currently in use are difficult to replace and they are not adequately equipped to cope with high impact conditions frequently experienced as the result of machine malfunctions. The present invention through a relatively minor modification of the existing bit holder achieves significant improvements in the ability of the holder to sustain such impacts without experiencing permanent damage and at the same time facilitates the removal and replacement of a damaged bit holder.